doc: add documentation

[larsesser]

Maybe, we could use some of the documentation of the voting functionality of the CdEDB. Anyway, I think we should provide a more detailed explanation of the (correct) handling of the library and the possibilities which it covers.

[larsesser]

I think this would also be a great usecase for doctests as this would kill two birds with one stone (some primitive tests and basic examples) although the latter is already covered by the tests.